1. Field of the Invention
The present invention relates to a radiographic image processing device and method which performs image processing including a scattered radiation removal process for a radiographic image and a program which causes a computer to perform a radiographic image processing method.
2. Description of the Related Art
In the related art, during the capture of a radiographic image of a subject using radiation that is transmitted through the subject, particularly, if the thickness of the subject is large, the radiation is scattered in the subject and the scattered radiation (hereinafter, also referred to as a scattered ray) causes a reduction in the contrast of the acquired radiographic image. For this reason, in some cases, when a radiographic image is captured, a scattered radiation removal grid (hereinafter, simply referred to as a grid) is provided between a subject and a radiation detector which detects radiation and acquires a radiographic image such that the scattered radiation is not emitted to the radiation detector. When imaging is performed using the grid, radiation which is scattered by the subject is less likely to be emitted to the radiation detector. Therefore, the use of the grid makes it possible to improve the contrast of the radiographic image.
In contrast, when imaging is performed using the grid, a subject image and a fine stripe pattern (moire) corresponding to the grid are included in the radiographic image, which makes it difficult to see the image. For this reason, a process is known which removes a stripe pattern caused by the grid from a radiographic image.
The grid has a structure in which radiopaque lead and a radiolucent interspace material, such as aluminum or fiber, are alternately arranged at a fine grid density of, for example, about 4.0 lines/mm. Therefore, the grid is weighty. For this reason, in portable radiography which is performed in, for example, a hospital room using a visiting car equipped with an imaging device, the grid needs to be provided between a lying patent and a radiation detector, which causes an increase in the burden of an arrangement operation on a radiographer and an increase in strain on the patient during imaging. Further, in the case of a convergence-type grid, density unevenness is likely to occur in the radiographic image due to the oblique incidence of radiation. In addition, a subject image and a fine stripe pattern (moire) corresponding to the pitch of the grid are recorded on the radiographic image, which makes it difficult to see the radiographic image.
For this reason, a process has been proposed which captures a radiographic image, without using a grid, and gives an image quality improvement effect, which can be obtained by removing scattered radiation using a grid, to the radiation image through image processing on the basis of imaging conditions (see U.S. Pat. No. 8,064,676B, JP1994-014911A (JP-H06-014911A), and C. Fivez et al., “Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation”, 1996, IEEE, pp. 339-342). The methods described in U.S. Pat. No. 8,064,676B and C. Fivez et al., “Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation”, 1996, IEEE, pp. 339-342 decompose a radiographic image into a plurality of frequency components, perform a scattered radiation removal process of controlling contrast or latitude for a low-frequency component which is regarded as a scattered radiation component, and combine the processed frequency components to acquire a radiographic image from which the scattered radiation component has been removed. In the method described in U.S. Pat. No. 8,064,676B, the scattered radiation removal process is performed by multiplying a low-frequency component by a gain corresponding to the hierarchy of the low-frequency component and the pixel value of the low-frequency component. Here, the gain is less than 1. The gain has a smaller value in a lower frequency band and is reduced as the pixel value increases. The method described in C. Fivez et al., “Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation”, 1996, IEEE, pp. 339-342 uses a table for converting a low-frequency component according to the pixel value thereof. In the method, lower frequency bands are increasingly reduced in a geometric progression manner.
According to the methods described in U.S. Pat. No. 8,064,676B, JP1994-014911A (JP-H06-014911A), and C. Fivez et al., “Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation”, 1996, IEEE, pp. 339-342, since no grid is required during imaging, it is possible to reduce strain on a patient during imaging and to prevent the deterioration of image quality due to density unevenness and moire.
In addition, the following method is known: when a radiographic image of a subject is captured using radiation that is transmitted through the subject, the radiation is more likely to be scattered in the subject and radiation transmittance becomes lower as the thickness of the subject increases, which results in a variation in the quality of the acquired radiographic image. For this reason, a technique has been proposed which roughly estimates the thickness of a subject, on the basis of various kinds of information, such as imaging conditions, a signal value of a radiographic image, the width of the histogram of the signal value of the radiographic image, and the length of the subject in a predetermined direction in the radiographic image and changes the conditions of imaging processing, such as a scattered radiation removal process for the captured radiographic image, or imaging conditions applied to capture a radiographic image, on the basis of the estimated thickness of the subject.
For example, JP1990-244881A (JP-H02-244881A) discloses a method which measures pixel values of a radiograph image of a simulated subject with a known thickness that is captured under known imaging conditions, prepares a correspondence table in which a body thickness is associated with the pixel value in advance, roughly estimates a body thickness distribution on the basis of the pixel value of the radiographic image with reference to the correspondence table, estimates a scattered component of the radiographic image corresponding to the body thickness distribution of the radiographic image, and subtracts the scattered component from the radiographic image to acquire a processed image.
In addition, Trotter et al., “Thickness-dependent Scatter Correction Algorithm for Digital Mammography”, Proc. SPIE, Vol. 4682, May 2002, pp. 469-478 discloses a method which estimates a scattered component of a radiographic image on the basis of a human body thickness distribution and removes the scattered component. The method disclosed in Trotter et al., “Thickness-dependent Scatter Correction Algorithm for Digital Mammography”, Proc. SPIE, Vol. 4682, May 2002, pp. 469-478 applies a predetermined function to an input radiographic image on the basis of the body thickness distribution estimated from the pixel value of the radiographic image to generate an estimated scattered radiation image, which is obtained by estimating the image of scattered radiation included in the radiographic image, and subtracts the estimated scattered radiation image from the radiographic image to generate an estimated primary radiation image which is obtained by estimating a primary radiation image from the input radiographic image. In addition, the method repeatedly performs the process which applies a predetermined function to the generated estimated primary radiation image to generate the estimated scattered radiation image and subtracts the estimated scattered radiation image from the radiographic image to generate the estimated primary radiation image until the estimated scattered radiation image is converged under predetermined convergence conditions, calculates a converged estimated scattered radiation image, and subtracts the estimated scattered radiation image from the radiographic image to finally obtain a processed image from which the scattered component has been removed. In addition, C. Fivez et al., “Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation”, 1996, IEEE, pp. 339-342 discloses a method which adjusts a predetermined function for estimating the image of scattered radiation included in the radiographic image according to the body thickness.
In contrast, in a general radiography system, when the imaging procedure (for example, a part of which the image is to be captured, an imaging direction (front or side), the purpose of radiologic interpretations, imaging conditions, the target of a radiation source, and the type of filter) of a patient, who is a subject of which the image is to be captured, is input to the system, default imaging conditions (for example, a tube voltage, an mAs value (that is, tube current×exposure time) which is a tube current-time product), and a source-image receptor distance (SID)) are set in the radiation source. When there are no problems in the default imaging conditions, the system captures an image using the imaging conditions to acquire a desired radiographic image. Therefore, a table in which default imaging conditions are associated with various imaging procedures is stored in the system. During imaging, imaging conditions corresponding to a designated imaging procedure are set and imaging is performed. In addition, the imaging procedure is stored together with a radiographic image. Therefore, in a case in which, for example, imaging processing is performed for the radiographic image after imaging, it is possible to acquire the imaging conditions when the radiographic image is acquired, with reference to information about the imaging procedure which is stored together with the radiographic image.